1. Field of the Invention
This invention relates to the fabrication of slider-mounted thin film magnetic read/write heads and particularly to a method for forming a slider air bearing surface (ABS) suitable for sub-nanometer clearances.
2. Description of the Related Art
In magnetic hard disk drives (HDD) the data on a disk is recorded and retrieved by a transducer (or “element”). Each magnetic head element is embedded within a slider which is in aerodynamic flight over a rotating disk at a height (flying height) of 10 nm or less. Other than passive fly height (fly height without active operator intervention), the active spacing during read/write is actually becoming even less, perhaps as low as 2 nm, in order to achieve higher areal density and disk capacity. For example, so-called fly on demand (FOD) or dynamic fly height (DFH) is now applied to control magnetic spacing via the protrusion of an element/transducer caused by the thermal effects of a locally embedded heater.
Under conditions of such low clearance, a variety of proximity interactions, such as intermolecular forces (IMF), meniscus and electrostatic forces (ESF) as well as the influence of disk topography, tend to destabilize the air-bearing slider. Moreover, operating shocks, lubricant transfer, altitude variations, thermal variations and even humidity, all become additional perturbations that play an increasingly critical role in determining the aerodynamic stability and, therefore, the reliability of the slider. It has proven to be very difficult to find an appropriate design of the air-bearing surface (ABS) of a slider that will enable it to fulfill the stability requirements of current operational conditions.
More specifically with respect to operating shock, there are two major failure modes, liftoff and compression, which are induced by the loss or gain, respectively, of suspension preload on the slider. With current negative pressure air-bearing surface design, the slider is more susceptible to liftoff resistance than compression resistance. In other words, the anti-liftoff is usually much better than anti-compression and, as a result, heads now tend to fail as a result of compression shock, given the sub-nanometer clearance of current operational conditions. Furthermore, in the conventional air-bearing surface designs, compression shock is fully coupled with Z-height sensitivity, which involves drive assembly tolerances. Good Z-height sensitivity used to hurt compression shock performance, which is why drive suppliers, for now, most likely rely on suspension optimization to obtain improvement in shock performance.
As concerns lubricant transfer, typically there is a layer of lubricant between the disk (or media) surface and the slider. When the slider glides above the disk surface, the lubricant experiences a very high air-bearing pressure which, given the negative and positive pressures that occur during operation, produces lubricant depletion and pickup. The majority of the lubricant that is transferred from the disk to the slider resides in the trailig edg (TE) of the slider, since it is the TE that flies very close to the disk surface. With backflows, the lubricant then migrates forward (towards the leading edge (LE) of the slider), accumulating in the airflow stagnation region. Experiments continue to indicate that the backflows play an important role in lubricant migration from the TE of the slider to the center pad where the transducer is located. As a result, the read/write clearance cannot stabilize because the accumulating lubricant on the ABS affects the aerodynamics of the slider and further affects its stability and the reliability of the HDD.
Regarding the sensitivity of the slider to ambient conditions, most of the existing sliders are capable of acceptable altitude performance with the help of an on-board pressure sensor. However, in order to minimize the compensation error for different tracks, the uniformity of altitude loss across tracks is becoming more important than the altitude loss itself. The impact of thermal variations on slider crown and pitch static angle (PSA) becomes evident as yet another concern, since even minor variations in clearance caused by thermal crown and PSA will have a significant impact on read/write performance in the sub-nanometer range. As a result, air-bearing designs with less sensitivity to crown and PSA is highly recommended, particularly at the inner disk (ID) where disk distortion and waviness are usually more severe and prone to modulate with slider ABS topography.
Referring to schematic, prior-art FIG. 1, there is shown a portion of a hard disk drive (HDD) in which an encapsulated, small thin film magnetic read/write head (30), embedded in a ceramic substrate called a slider (7), is used to read and write data on a magnetic medium or storage disk (15). The read/write head (30) is formed using well known semiconductor deposition techniques such as electroplating, CVD (chemical vapor deposition) and photolithographic patterning and etching.
The slider (7) has a backside surface at which it attaches to the distal end of a head gimbal assembly (HGA) (150) and a patterned air-bearing surface (ABS) typically referred to as an ABS plane (300) that faces the rotating disk (15) during HDD operation. The ABS plane (300), shown here in a side view, only defines a virtual surface boundary of the patterned slider ABS, because the patterning actually incises the plane to create channels for airflow and to control the aerodynamics. These channels will be shown in later figures and cannot be seen in the side view here. The (HGA) (150) is activated by an electro-mechanical mechanism and electronic control circuitry to position the slider-mounted head at various positions along the magnetic tracks on the disk (tracks not shown).
As the disk is rapidly rotated by a spindle motor (not shown), hydrodynamic pressure causes an air flow (arrow, (25)) between the patterned ABS plane (300) of the slider (7) and the surface of the disk (15). This flow lifts the slider so that it literally flies above the surface of the disk at a “fly height” (80), supported on a layer of air. This fly height, in the prior art, is approximately 10 nm or less. The edge of the slider into which the disk rotates is called its “leading edge” (200), the opposite edge, which contains the read/write head (30), is called the “trailing edge” (100). The aerodynamics of the slider motion lifts the leading edge higher above the rotating disk surface than the trailing edge.
As schematically illustrated in FIG. 2, the surface of the disk (15) is typically covered by an irregular layer of lubricant, which is indicated here as small spherical beads (150). The beads cluster irregularly into small mounds or “moguls” (155). The airflow, indicated by arrow (25), beneath the ABS of the slider creates a distribution of air pressure against the ABS, which is schematically indicated by a curve (160). There is also shown the flow of lubricant (170) to the trailing edge surface (100) of the slider. The flow of lubricant to the trailing edge surface of the slider is a result of the very high air-bearing forces that act on both the ABS of the slider (as indicated by the pressure curve) and on the distribution of lubricant on the disk surface. These forces are instrumental in forming the lubricant moguls (155) as well as the lubricant transfers (170). The detailed causes of the lubricant behavior depend on both the forces and the lubricant properties, such as molecular roughness and cohesion. With the effect of negative pressure ABS design (to bring the slider close to the disk surface), the majority of the lubricant transferred to the slider is sucked beneath the trailing edge (100) of the slider ABS by a backflow of air that is oppositely directed to the arrow (25). The lubricant can be sucked into an air pocket within the patterned ABS of the slider (see FIG. 8B), where it accumulates and resides, because of the sub-ambient pressure within it. Elimination of this lubricant accumulation will be one of the objects of the present invention.
Now, however, there is a need in the HDD industry to employ a new concept in air-bearing surface design that can aid in sub-nanometer clearance applications. The role of ABS topography in improving slider performance is well known in the prior art and such a prior art slider was shown above.
Sannino et al. (U.S. Pat. No. 6,515,831) teaches a slider surface with channels for enhanced damping and a subambient pressure cavity. (US Publ. Patent Appl. 2009/0219651) teaches an air-bearing design for HDD applications. Dorius (US Pat. Appl. 2010/0128395) discloses a T-shaped channel with a bar and a lubricant accumulation barrier at its trailing edge. Pendray et al. (U.S. Pat. No. 6,989,967) shows a T-shaped center channel and side recesses that are not closed at the trailing edge. Bontaghou et al. (U.S. Pat. No. 6,809,904) shows a center cavity and two side dams at the trailing edge. Rajakumar (U.S. Pat. No. 7,245,455) discloses closed dams at two sides near the trailing edge. In addition, the role of the suspension in reducing the effects of shock is also known in the prior art. In this regard, Mei et al., (U.S. Pat. No. 7,706,106) teaches a hard disk drive suspension lifter with reinforcing features for high shock resistance. None of these prior art teachings address the problems to be addressed by the present invention.